One of the earliest Christian hymns outside the Bible is "Phos Hilaron" ("Hail Gladdening Light"), dating back to the 3rd or 4th century. Phos: (φῶς) is the Greek word for light. Hilaron: (ἱλαρόν) is an adjective meaning joyful, gladsome, gracious, or gentle. It's still used in Orthodox Vespers today, and is used as the Vespers hymn at OLP, in the Byzantine English rendition. 

The Latin rendition, Lumen Hilare, dates to the 4th Century.